[Campbell Biology P.426] The 'VISUAL SKILLS' prompt asks about the enhancer for the albumin ... | Practice Question

The 'VISUAL SKILLS' prompt asks about the enhancer for the albumin gene. Considering the information in Figure 18.12 and its accompanying text, which statement accurately describes the albumin gene enhancer and its function in different cell types?

  • A: The nucleotide sequence of the albumin gene enhancer is identical in both liver and lens cells, but its activity is dependent on the presence of specific activator proteins.
  • B: The albumin gene enhancer has a different nucleotide sequence in liver cells compared to lens cells, allowing cell-type specific expression.
  • C: The albumin gene enhancer is only present in liver cells, explaining why it's not expressed in lens cells.
  • D: The enhancer functions independently of activators, with its activity solely determined by its proximity to the promoter.

Explanation

The 'VISUAL SKILLS' question prompts us to consider the enhancer sequence. The text states that 'Both liver cells and lens cells have the genes for making the proteins albumin and crystallin.' This implies that the entire genetic locus, including the enhancer, is present in both cell types and thus has the same nucleotide sequence. The cell-type specific expression is not due to a different enhancer sequence but to the 'unique combination of elements' within the enhancer and, crucially, the differential presence of specific 'activator proteins required for high-level expression' in each cell type. Therefore, the enhancer's activity is contingent on the availability of these specific activators, not on its own sequence changing.